at t = 2 hours, N = 0.80 N₀

0.80 = e^{-l_eff 2}

l _eff = ln0.80/(-2 hr) = 0.1116 h^-1
 

Also; l _p = ln2/T_p = ln2/(4X24 hr) = 7.22X10^-3 h^-1
 

l _eff = l_p + l _b

0.1116 = 7.22X10^-3 + l _b

l _b = 0.1046 h^-1

T_b = ln2/l _b = ln2/(0.1046 h^-1) = 6.64 h = 0.28 d

Answer is C

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